Production of pulp, etc.



Patented Nov. 3, 1931 UNITED STATES PATENT OFFICE 4;

LINN BRADLEY, 0F MONTCLAIR, NEW JERIElY, AND EDWARD P. MCKEEFE, OF NEWYORK, N. Y., ASSIGNORS T0 BRADLEY-MCKEEFE CORPORATION, OF NEW YORK, N. Y

A coaronmron on NEW YORK rnon'uc'rron or PULP, me.

No Drawing. Application filed June 28,

' This invention relates to improvements in the production of wood pulpfrom wood and similar cellulose-bearing materials, whereby materiallyincreased yields of improved pulp can be obtained, and whereby woodswhich are ordinarily considered of little or no value for the productionof pulp can be utilized for the production of high grade pulp.

The common methods of producing wood pulp chemically, are the so-calledsoda, sulfate and sulfite processes. In the so-called soda process, themain cookingoperation is dependent upon caustic soda. In the so-calledsulfate process the main cooking operation is dependent upon causticsoda assisted more or less by sodium sulfide, although some sodiumcarbonate and sodium sulfate may be present. In the so-called sulfiteprocess the main cooking operation depends upon bisulfites in thepresence of sulfurous acid. In the soda and sulfate processes,accordingly, the cooking liquor is strongly alkaline, while in thesulfite process the cooking liquor is strongly acid; and it has beenconsidered impractical to cook woodsuccessfully, for the production ofchemical pulp, unless the cooking liquor was either strongly acid orstrongly alkaline.

In all three of the processes mentioned, the cooking liquors employedact not only upon the encrusting and so-called non-celluloseconstituents or so-called lignine .or lignified .tissues of the wood,but also to a greater or less extent upon the cellulose or pulp making.

fibres, and usually destroy or injure a considerable portion of thevaluable fibres .or cellulo e, or convert an appreciable portion thereofinto constituents which are removed from .the pulp. Sometimes theresidual liquors from the process are also of such a character that theyleave in the pulp constituents which make the pulp diificult to bleachwith the usual bleaching materials and operations, so that the fibresmay be still further injured by the severe bleaching treatment which itis nece sary to employ.

a The rapidly decreasing supply of woods suitable and availablefor pulpmaking purposes and the high costof equipment and operation of chemicalpulp mills makes it of 1921. Serial No. 481,147.

great national and economic importance to materlally increase the yieldof pulp from woods which are commonly used for the purfor making groundw d by utilizing a' cer-,

tain limited class of trees and using hydraulic power to grind suchwoods, is obviated, this making available a large amount of hydraulicpower for other industrial uses. The pulp obtained by chemical methodsof disintegrate mg wood even those now employed, is far superior inquality, including strength, to

that obtained by the ground wood process,

and a much wider range of woods can be used.

As the result of our investigations, we have been impressed with thefact that a considerable portion of the pulp content of the wood treatedwas not being recovered in the form of pulp and we have thereforestudied the cooking processes with the view of determining whether anincreased yield of pulp could be obtained from the same amount of woodby other chemical methods than those heretofore employed.

In the course of our investigations, we

modified the ordinary soda process by substituting neutral sodiumsulfite for part of the caustic soda commonly employed, so that 85 thecooking liquor was a composite cooking liquor containing both neutralsodium sulfite and caustic soda, for example, in equal proportions, andwe found that an improved result was obtained as compared with theordinary soda process.

'In the course of we omitted entirely the caustic soda from the cookingliquor in order to determine the effect of a neutral sodiumsulfite'liquor when used as' the cooking liquor; and we have found thatnot only could a greatly increased yield of wood pulp be obtained, butthat the resulting wood pulp was of a superior character. This discoverywas the more surprising as it has heretofore been considered essentialto our further investigations use either a strongly caustic or acidicliquor for producing-pulp from wood by a chemical cooking treatment. Ourinvestigations showed further that the neutral sodium sulfite cookingliquor had no such injurious action upon the desired pulp fibres as astrongly acid or strongly alkaline cooking liquor, and that, even whenthe amount of sodium sulfite employed was double the strength of thereagents commonly employed in other processes, there was noobjectionable effect upon the fibres produced or the pulp yield.

Our investigations have also shown that there are greater or lessdifferences in the various kinds of wood. but we have found that woodscan be divided, generally, into three classes. namely, (1) those woodswhich contain little or no 'resinous constituents, such as poplar.birch, cotton. wood. aspen, bass wood and similar woods: (2) woods whichcontain moderate amounts of resinous constituents. such as spruce, fir.hemlock, balsam and the like: and (3) woods which contain relativelvlarge amounts of such constituents such as Georgia pine. white pine.iack pine, long leaf pine and other similar pines.

We have found that woods of the first class can be treated in asatisfactory and advantaneous wav with the normal sodium sulfite cookingliquor alone. and without the addition of caustic soda. inasmuch as suchwoods contain little or no resinous or other .constituentsfother thanthe fibres themselves. which are not acted upon bv the hot solutions ofsodium sulfite. For cooking woods of the second class we have found itadvantageous in some cases to use a small or moderate amount of causticalkali. e. g. sodium bydro ide. near the end of the cook. in additiontothe required amount of normal sodium sulfite. We have found that amoderate amount of caustic alkali is of great assistance near the end ofthe cook in removing or dissolving certain constituents. such asresinous constituents. associated with the fibres. but that the amountof caustic alkali requ red for this purpose is suflicientlv small sothat there is substantially no injurious action upon the vield of thefib es. In treating woods of thethird class. we have founditadvantageous to add an amount of caustic alkali. near the end peratureand pressure, .for example, corresponding to saturated steam at 60 to180 pounds per square inch, whereby the cooking operation is carried outwithout any objectionable injury to the fibres and with the prgductionof an increased yield of superior pu p. O I

If wood chips are cooked under pressure with water alone, for example,at about 120 pounds per square inch of saturated steampressure, more orless disintegration of the wood takes place and considerable amounts oforganic matter are found in the solution at the end of say 4-. or 5hours treatment at such temperature and pressure; and the solution isacid in reaction to litmus. If wood chips are cooked with only a smallamount of sidium sulfite, we have, found that the liquor will alsobecome acid during the cooking operation and asatisfactory pulp will notbe produced without subsequent treatment; but we have found that if asufficient amount of sodium sulfite is present during the cookingoperation, a satisfactory pulp can be readily produced. The sodiumsulfite appears to combine with the constituents which otherwise wouldgive an acid reaction, thus preventing the development of an acidcondition during the cooking operation. The amount of sodium sulfiterequired varies somewhat with different woods but a minimum of at leastabout 30% seems necessary with most woods, and with some woods a minimumof at least about 85% or somewhat more appears necessary. For example,in treating old poplar wood containing about 20% of water, we have foundthat an amount of sodium sulfite corresponding to about 12 or 15 or even20% of the air-dried weight of the wood (i. e. wood containing 10% ofwater) did not completely 'cook the chips and gave a residual liquoracid in reaction; while with about 25% of the normal sodium sulfitebased on the air-dried weight of the wood the'liquor was about neutral;and with 30% or more of sodium sulfite no difliculty was had inobtaining a satisfactory pulp and avoiding an acid reaction in thecooking liquor. We have increased the amount of sodium sulfite up to ashigh as 45 and 50% of the air-dried 7 Per cent Old poplar 30 to 35 Oldbirch 30 to 40 New poplar or birch 30 to 40 Pine or spruce 35 to 45Hemlock 30 to 40 It is one advantage of the present process not haveinjurious action upon the fibres. It

is possible. accordingly, to use a more concentrated cooking liquorwithout injury to the fibres due to the concentration.

In carrying. out the present process, gesters of iron or steel are welladapted for use, as the cooking liquor employed is inert to thesematerials. It is unnecessary to provide the digesters Wit-h anacid-resisting lining, such as is now commonly provided in'digestersused in the sulfite process, and, where the digester-s contain such a lining, it can be removed, thus correspondingly increasing the capacity ofthe digester.

The normal sodium sulfite utilized in the present process, can beobtained from any '20 suitable source of supply. It is an advantage ofthe process that the sodium content can be recoveredif desired from theresidual liquors for reuse, thus eliminating the necessity for acontinued fresh supply of this material other than that required to makeup for losses.

With woods such as poplar, birch and hemlock, the entire cookingoperation can be carried out with the normal sodium sulfite cookingliquor and without the presence of canstic soda either during or nearthe end of the cooling operation. WVit-h resinous woods such as spruceand pine the normal sodium sulfite cooking liquor will remove thegreater part of the non-fibrous constituents but it leaves certain ofthe resinous and similar constituents with the fibres. For some purposesthe presence of such constituents in the resulting pulp may beunobjectionable, or-

even advantageous, as they may supply a certain amount of sizing; butthe pulps-containing such resinous constituents are darker in color,and, for many purposes, are less valuable than with the resinousconstituents removed. We have found that such resinous constituents canbe readily removed by the use of a small amount of caustic soda near theend of the-cooking operation, that-is, after the main portion of thecooking operation has been carried out with the normal sodium sulfitecooking liquor. If the canstic soda is added at the beginning of theprocess, a darker pulp and residual liquor, are obtained, and thepresence of caustic alkali during the early portion of the cooking seemsto be without advantageand even obj'ectionable in some cases, although asmall amount of caustic alkali can be tolerated. But We have found thatifthe caustic alkali is used after the wood has been subjected to .theaction of the normal sodium sulfite cooking liquor, it will act upon theremaining.

ortions of the wood and will free the fibres om the resinousconstituents which the sulfite liquor itself has not removed. Bycarrying out a two-stage cook of this character, accordingly, we havefound it possible to produce a light colored and. easily bleachable pulpwith the use of only a small amount of caustic soda added near the endof the cooking operation or utilized as a subsequent cookingliquo-rafter the residual liquor from the sodium sulfite cook has been removed.

Resinous woods can also be subjected to a cooking operation witha-normal sodium sulfite cooking liquor until the cooking operation isnearly finished, the charge can then be blown from the digester andscreened and the screenings treated with dilute caustic soda solutiono-rthe pulp can be washed with dilute caustic soda solution to removeresinous constituents which have not been acted upon and removed by thesodium sulfite cooking liquor.

The knots in coniferous woods, which frequently give difficulty in theordinary acid sulfite process do not appear to interfere in the carryingout of the present process and they are acted upon in much the samemanner as the other parts of the wood so that a substantially uniformpulp is produced. The

of the non-fibrous constituents therefrom.

During the carrying out of the digestion,

when resinous woods are treated, the turpentine can be removed from thedigester by bleeding the digester, that is, by permitting part of thegases to escape from the digester at successive time intervals. Thecooking operation can be somewhat hastened by using higher temperaturesand it is an advantage of the present process that it permits highertemperatures to be used without objectionable injury {to the resultingfibres. For example, .at temperatures corresponding to steam pressuresapproaching 140 pounds, e. g}, between 130 and 140 pounds,

the time required for the cooking operation can be reduced to around 3hours or somewhat more, thus greatly increasing the number of cookswhich can be completed in a single digester per day, and correspondinglyincreasing the output of the plant. Pressures materially in excess of140 pounds may also be used, e; g., saturated steam pressures up to.-around 180 pounds, as hereinbefore pointed out. 7 x

The present process also makes more feass ible the cooking of mixedwood, that is, mixtures of different woods of a like character at thesame time, as prolonged cooking of one of the woods until the other woodis 5 sufficiently cooked does not injure the fibre.

The cooking process of the present invention seems comparable to anextraction procoss in which the non-fibrous constituents of the wood areextracted without objectionable action or injury to the fibres, so thatthe fibres are obtained in a substantially uninjured condition. 4

The fibres produced by the present process have the appearance andstructure of uninjured fibres. The fibres thus have a characteristicsheen or silky appearance much like that of the original wood. The pulpmay therefore be considered to contain practically all of the fibres ofthe original wood freed from non-fibrous substance and in asubstantially uninjured condition.

The yield of pulp obtained by the process of the present invention isalso radically higher than is commonly obtained by the ordinary chemicalpulp methods. We have found that the yield of pulp can be increased asmuch as 25% to over that commonly obtained, for example, by the ordinarysoda process from poplar wood. We have found that the ordinary yieldsof, for example, 38 to of pulp from poplar wood can be increased to ashigh as 60% or higherof pulp, figuring the pulp on an air-dried basis,(i. e. 10% of H 0). We have also found that a greatly increased yieldcan be obtained in the case of spruce wood and pine wood, when treatedby the present invention, as compared, for example, with the ordinarysulfite process. With old poplar Wood, we have obtained a yield ofaround 61%of pulp based on the air-dried weight and using an amount ofsodium sulfite equal to 30% of the weight of the air-dried wood and wehave obtained the same yield with an amount of sodium sulfite equal toof the air-dried wood; with hemlock we have obtained a yield of pulpequal to about 50% on the air-dried basis; with new poplar wood, a yieldof about 67%; and with new birch, a yield of about It is one advantageof our process that heavy woods, such as birch, beech, locust,

southern pine, tamarack, etc., give more pulp per cord and per digestercharge than do the lighter woods so that a correspondingly less amountof liquor and less steam can be safely used than with the caustic sodaand sulfate processes. I

The pulp produced'by the present invention is different in characterfrom the pulp commonly rpoduced by the present methods in that thefibres are obtained in a substantially uninjured condition, and weaccording regard the pulp as a new and improved pulp. It contains longfibres possesslng great strength. Georgia pine wood, spruce and strongpaper from poplar pulp without the hemlock for examples, when treatedaccordin g to the present invention, and after bleaching, yielded anexcellent snow-white fibre of great strength and beautiful lustre.

Inasmuch as different woods differ in their 7 fibrous structures, itwill be evident that the pulps produced therefrom will differ somewhatfrom each other. The process of the present invention accordingly willwe different kinds of pulp with different ds of fibres, each fibre beingmore or less characteristic of the wood from which it is produced.

Owing to the differences in the pulp itself, it is possible to use thepulp for purposes for whichpulp from the same kinds of wood could notheretofore be used. For example, it is customary in the art to mix theshort fiber pulp made from poplar wood by the soda process with the longfibre pulp made from spruce by the so-called sulfite process, in orderto obtain a paper of suitable strength. The present invention, however,makes possible the production of a stronger paper from poplar fibres(made by treating poplar wood according to the present invention) thathas heretofore been commonly made from a mixture of poplar fibres madeby the soda process, and spruce fibres made by the so-called sulfiteprocess. In other words, the present invention gives a pulp from shortfibre wood such as poplar which possesses many of the properties of pulpheretofore made from longfibre wood. The present invention accordinglymakes possible the production of a necessity of obtaining and admixinglong fibred pulp.

The present invention is also applicable to the production of a highquality of pulp from various species of wood which heretofore have beenconsidered of little or no value for the manufacture of high grade pulp.In particular, we have found that resinous woods, such as pine wood, canbe advantageously treated by our improved process without difficultyarising from the resinous constituents which the woodcontains,particularly if a small amount of caustic alkali is used during thelatter part of the cooking 116 operation. In general our new process isapplicable to the various woods now commonly treated for the manufactureof pulp and in addition to the treatment of other woods, both coniferousand deciduous, which 120 heretofore have-notbeen considered availablefor the production 'of high grade pulp. We have found further that theresidual liquors obtained from one cook can be used over again, with theaddition thereto of a further as five successive charges. The presentprocess, accordingly, is well adapted for use in a plant having a seriesof digesters and with the treatment of successive charges with the samecooking liquor, with suitable additions of sodium sulfite thereto. Thisrepeated use of the cooking liquor, with suitable additions ofsodiumsulfite, can also be used to ad amount of caustic soda.

The residual liquors produced by the present process are ofcharacteristic composition and properties and different in color,consistency, composition, reaction with acids, etc., from the residualliquors from the soda or sulfite or sulfate processes, when the likekinds of woods are treated. They arefree, or substantially so, from theproducts of degradation of cellulose, inasmuch as the cellulose of thefibres is little, if any, acted upon by the present process. Theresidual liquors are usually of a mahogany color which may vary inintensity. In consistency, they are thinner than the so-called blackliquors from the soda and sulfate processes. When the residual liquors,for example, from poplar wood, are treated with acids, to give an acidreaction, the color is discharged, although the color is again developedor restored on again making the liquor alkaline in reaction. Theacidified residual liquor can be filtered and will then contain lessorganic matter than the corresponding black liquor from the sodaprocess, The organic constituents of the residual liquors differsomewhat, according to the wood treated. With different woods treated,accordingly we obtain somewhat different characters of by-products, andof precipitated matter upon acidification. It will thus be evident thatdifferent by-product-s can be obtained from the residual liquorsresulting from the treatment of different woods,

and that the residual liquors can be treated ac cording to variousmethods of treatment for the recovery of these by-products. The residualliquors can advantageously be treated for the regeneration of the sodiumsulfite, for further use in the process.

lVhen the same liquor is used repeatedly for successive cookingoperations, with successive additions of sodium sulfite thereto beforeeach successive cook, the amount of dissolved matter in the liquorjwillbe greatly increased until finally the liquor becomes practicallysaturated. The resulting liquor will be of a much more concentratedcharacter and will be correspondingly more valuable,

for example, for recovering the by-products' or sodium sulfitetherefrom, inasmuch as there is less water present and less liquor toevaporate or to handle.

The present invention'will be further illustrated by the following moredetailed description of certain specific applications thereof; v

Poplar wood which has been partly dried or seasoned and from which thebark has been removed, and isas free as practicable from cinders, sandand other dirt and fungus, is chipped or broken into pieces aboutthreeeighths of an inch to an inch long, the chips are screened toremove sawdust and fine dirt and the screened chips are conveyed to thechip storage bin from which they are fed to the digester. should befairly uniform but it may range in practice ,up to about 30 percent ormore.

A suitable digester is one made of iron constructiom'and unlined, and ofa stationary vertical type, a welded digester being preferred to ariveted digester. The digester is preferably coverodwith a non-conductorThe water content of the chips I or heat insulating material forthepurpose of I proper strength and construction to with-' stand thepressure developed during the cooking and to permit the discharge of thecooked chips at the completion of the cooking operation. The digestermay be provided with a false bottom and with inlet and outletconnections, valves, etc. and may advantageous- 1y have a suitablevertical pipe arranged therein through which the cooking liquor may becirculated. The discharge pipe or outlet from the bottom of the digesterdelivers the cooked chips to a blow-pit usually situated at a levelabove that of the vats or other washing tanks in which the residualllquoris separated from the pulp and wherein the pulp is washed, so thatthe chips can be forced from the digester, at the end of the cook, bymeans of the residual pressure in the digester. The cooking liquor canbe supplled from a suitable storage tank therefor.

For a digester of about 8 feet in diameter and 25 feet high, about 4,000gallons or more of a suitable cooking liquor may be used for theair-dried weight of the wood. The chips may be charged into the digesterwhile it is still hot from the previous cooking operation, or the liquorand chips can be run together into the digester simultaneously. Whensufiicient chips and liquor have been run in to permit the establishmentof the circulation of liquor, steam can be turnedon both below the mainbody of chips and so as to direct the liquor upward through thecirculating pipe so that circulation of the liquor is established andmaintained while the chips are being run into the digester. When thedigester has been filled and closed, it is then brought up totemperature and pressure as rapidly as practical, using direct steam.The temperature and pressure are thereafter maintained fairly constant,for example, at pressure of about 120 pounds persquare inch, until thecooking operation is finished. The time required to bringthe digester tothe desired temperature and pressure may be,

for example, about one or two hours, and the cooking operation mayrequire from four to six hours at pressure for its completion, althougha longer or shorter period may be used. So also, the amount of sodiumsulfite may be somewhat increased or decreased from that aboveindicated, since even a large excess does not injurethe pulp. So also,different pressures may be used, e. g., from about 110 to about 140pounds per square inch, with a corresponding temperature (that is, thepressure will depend upon the temperature of the steam or waterwithinthe digester).

In order to promote circulation during the process of the digestion, itmay be necessary or advantageous to relieve the internal pressure of thedigester somewhat for a short period. This relief can be accomplished byopening a valve in the pipe line leading from the top of the digester tothe blow pit. When a suitable reduction in pressure has been obtamed,the valve is again closed and circulation of the cooking liquor withinthe digester continues, steam being fed in partly for this purpose.

When the cooking operation has been finished, the steam feed pipe isshut off and thepressure may be reduced to the proper point, forexample, 75 pounds per square inch, or even more and the valve in thepipe leadlng from the bottom of the digester to the blow pit is thenopened and the cooked chips and liquor are forced by the remainingpressure through said discharge pipe from the digester to the blow pit,this operation breaking up the cooked chips into pulp. The pulp andresidual liquor can then be run into vats or tanks: and the residualliquor separated more or less from the pulp, the

pulp then washed in the usual way, and the washed pulp then subjected tofurther treatment= such as is customary in the art.

The residual liquors can be separately treated for the production offurther amounts of cooking liquor therefrom.

We have found that a more concentrated poplar chips. The cooking may becarried out by heating under pressure to a temperature corresponding toa saturated steam pressure of about 120 pounds per square inch, eitherby direct introduction of steam, or by indirect heating and circulationof the liquor by means of pumps so that the cooking liquor is heated bya suitable heater outside the digester and circulated through thedigester, preferably so that the cooking liquor is spread out at the topof the digester and caused to percolate down through and around thechips. In this way the chips can be cooked without being completelysubmerged; while the nature of the cooking liquor'enables it to behandled by pumps at high temperatures and pressures. Also, withcirculation of this kind the composition of the liquor can be changedduring the cooking operation.

In treating woods of the second class above described, for example,spruce wood, the p woods can be treated 1n a similar manner to causticsoda to the normal sodium sulfite cooking liquor during the latter partof the cook, so that the fibres may be readily separated from resinousand similar ingredients.

be prepared in the customary manner, but it is unnecessary to removeknots from the wood before chipping as knots can be successfully handledin the process of the present invention.

- Woods of the-third class above described can be treated in a similarmanner except that a somewhat larger amount of caustic soda isadvantageously added near the end of the cook, on account of the greaterpercentage which these woods contain of resinous and other ingredientswhich are removed only with difiiculty by a sodium sulfite cookingliquor alone.

For example, in treating highly resinous Georgia pine, which has beenair seasoned, the wood can be freed from bark and chipped and the chipscooked in a digester for a suitable period (for example, about 4 hoursor more) at a temperature corresponding to a saturated steam pressure ofabout 120 pounds per square inch, using a cooking liquor containing anamount of normal sodium sulfite equal to about 35 to 40% of the weightof the Wood (figured on an air-dried basis); then removing the largerportion of the cooking liquor from thedigester, and introducing afurther amount of cooking liquor containing a few percent of causticsoda (e. g.,' 3 to in addition to the sodium sulfite (e. g.,

. 20 to 25%) and cooking the chips at a temtheir characteristic markingsto a high degree. The bleached pulp has a lustre or sheen such as thatpossessed by the freshly broken chips, thus indicating little, if any,damage to the fibres. A high yield of pulp is also obtained.

It will thus .be seen that the present invention includes anew method ofcooking Wood for the production of pulp therefrom without objectionableinjury to the cellulose fibres, so that an increased yield of a new andimproved pulp is obtained, which will differ somewhat with differentwoods treated, and that the process is of more or less. generalapplication to the treatment of coniferous and deciduous woods such as"poplar, spruce, balsam; hemlock, pines, birch,

basswood, beech, maple, etc. It will also be seen that the process ofthe present invention requires few changes in, existing mills for it'sinstallation. Sulfite 'mills, so-called, can beggreatly'simplified, andthe capacity of the digester increased by the elimination and removal ofthe lining therefrom, and by the fact that sulfurous acid recoverysystems, such as are usually,employed,'are'not required.

It will be' evident that the present invention presents many advantages.Among these advantages may be briefly mentioned the following: Thecooking liquors can be prepared and the residual liquors obtainedeconomically; objectionable odors such as are produced by the sulfateprocess are avoided; digester linings, such as are commonly used in theso-called sulfite process, are not required, and can be. removed, thusincreasing the effective size of the digester and materially increasingits capacity; the pulp produced containsthe fibres of the wood in apractically unchanged condition except for the removal ofencrusting andnonfibrous constituents therefrom; the-yield of pulp is very.materiallyincrea sed, while the residual liquors produced have acorrespondingly reducedcontent of wood constituents; the pulps,especially those from pines, can be advantageously bleached by a mixtureof hypochlorites and -permanganates; the residual liquors can beadvantageously treated for the regeneration or production of cookingliquor for reuse in the further carrying out ofthe process; and thecooking liquor is of more or less universal application to the treatmentof various different kinds of wood. I

We claim 1'. The method of producing wood pulp from resinous wood, whichcomprises subjecting the wood to a cooking operation at a pressure ofaround 110 to 140 pounds per square inch and at a correspondingtemperature with a cooking liquor containing an amount of normal sodiumsulfite eqiial to at least about 30 to 35% of the weight of the .wood(figured on an air-dried basis)."

2. The method of producing wood pulp from resinous wood, which comprisessubjecting the wood to a cooking operation under pressure and at atemperature corresponding to a saturated steam pressure of about 100pounds or higher per square inch with a cooking liquor consistingessentially of a solution of sodium sulfite, the amount of sodiumsulfite (-Na sO being in excess of about 30 per cent of the air driedweight of the wood.

3. The method of producing wood pulp from wood, which comprisespartially cooking the wood with a cooking liquor consisting essentiallyof a normal sulfite and completin the cooking operation with theaddition of caustic alkali.

4.=The method of producing wood pulp from wood, which comprisessubjecting the wood to a cooking operation under pressure and at anelevated temperature with a cooking liquor containing sodium sulfite,subsequently adding an alkali to the liquor and completing the cookingoperation.

5. The method of producing wood pulp from wood containing resinousconstituents, which comprises cooking the wood under pressure and at anelevated temperature with liquor from such cooking operation, adding afurther amount ofa cooking liquor containing a normal alkali sulfite andcaustic alkali and completing the cooking operation.

7. The method of cooking wood chips and knots of resinous woodssimultaneously, which comprises subjecting them to a cook ing operationunder a temperature corresponding to a saturated steam pressure of morethan 100 pounds with a non-acid cook ing liquor containing essentially anormal alkali metal sulfite, the amount of alkali sulfite, expressed asNa SO being more than 30 per cent and less than 50 per cent of the airdried weight of the wood.

8. The method of producing wood pulp from resinous wood which comprisessub jecting the Wood to a cooking operation at a temperaturecorresponding to a steam pressure in excess of 100 pounds, with acooking liquor containing a normal alkali sulfite, expressed as Na SOamounting to around 30 t9r50% of the air dried weight of the wood. \9.The method of producing wood pulp from resinous wood which comprisessubjeoting the wood to a cooking operation at a temperaturecorresponding to a steam pressure in excess of llO'pounds, with acooking liquor containing a normal alkali sulfite, ex-

pressed as Na SO amounting to around 30 to 50% of the air dried weightof the wood.

10. The method of producing woqd pulp from resinous wood which comprisessubjecting the wood to a cooking operation at a temperaturecorresponding to a steampressure of 120 pounds or higher, witha cookingliquor containing a normal alkali sulfite, express-2d as Na SO'amounting to around 30 to 50% of the air dried weight of the wood.

11. The method of producing wood pulp from resinous wood which comprisessubjecting the wood to a cooking operation at a temperaturecorresponding to a steam pressure around 130 to 140 pounds, with acooking liquor containing a normal alkali sulfite, expressed as Na SOamounting to around 30 to 50% of the air dried weight of the wood.

12. The method of producing resinous wood pulp from wood which comprisessubjecting the wood to a cooking operation un der pressure and at anelevated temperature with a cooking liquor containing essentially anormal alkali sulfite, the composition of the liquor being such as toavoid any strong in excess of 100 pounds with a cooking liquorcontaining essentially a normal alkali sulfite, partially reducing thepressure in the digester at the end of the cooking operation, and t enblowing the digester.

14. The method of producing wood pulp from wood which comprisessubjecting the wood to a cooking operation under pressure and at anelevated temperature with a composite cooking liquor made up in part ofresidual liquor from the cooking of wood with a normal-sodium sulfitecooking liquor and having added amounts of sodium sulfite combinedtherewith.

15. The method of producing wood pulp from wood which comprisessubjecting the wood to a cooking operation under pressure and at anelevated temperature with a cooking liquor containing essentially normalsodium sulfite, the concentration of the sodium sulfite in the liquorbeing from about 100 to 200'grams per liter.

16. The method of producing wood pulp from heavy woods, materiallyheavier than poplar, which comprises subjecting a digester charge ofsuch heavy wood to a cooking operation under pressure and at an ele-Vated temperature with a cooking liquor containing a normal alkalisulfite in amount. around 30 to 40% of the air dried weight of the wood,the liquor containing in excess of 100 grams per liter of the sulfite.

17. The method of producing wood pulp from resinous wood which comprisessubjecting the wood to a cooking operation at a temperaturecorresponding to a steam pressure in excess of about 110 pounds, with acooking liquor containing around 35 to 45 per cent sodium sulfite (Na SOon the air drIed weight of the wood.

' 18. The method of producing wood pulp from resinous wood whichcomprises digesting the wood with a liquor containing solublemonosulfite as substantially the only digesting reagent, the quantity ofmonosulfite used being sufiicient to continue the cook and the cookbeing continued until encrustingmaterials have been dissolved anddisintegrated andseparable wood fibers charact'erized by extraordinarystrength, flexi-' bility, felting and bleaching qualities have beenproduced substantially as described.

19. The method of producing wood pulp from resinous'wood which comprisesdigesting the wood with a liquor containing sodium monosulfite assubstantially the only digesting reagent, the quantity of monosulfiteused being sufficient'to continue the cook and the cook being continueduntil encrusting materials have been dissolved and disintegrated andseparable wood fibers characterized by extraordinary strength,flexibility, felting and bleaching quality have been producedsubstantially as described.

20. The process of producing chemical pulp from wood which comprisescooking the wood with a liquor containing essentially sodium monosulfitein the form of a liquor containing in excess of about 80 grams per literof sodium monosulfite, the amount of liquor employed being suflicient toleave an excess of sodium monosulfite in the liquor at the end of thedigestion. a In testimony whereof we aflix our signatures.

LINN, BRADLEY. EDWARD P. MCKEEFE.

